\documentclass[10pt]{article}
\usepackage[usenames]{color} %used for font color
\usepackage{amssymb} %maths
\usepackage{amsmath} %maths
\usepackage[utf8]{inputenc} %useful to type directly diacritic characters
\begin{document}
\begin{align*}Z = \tanh(f(X, \color{red}w_{1}\color{black}, \color{red}b_{1}\color{black}))\\
Y = \text{ReLu}(f(Z, \color{red}w_{2}\color{black}, \color{red}b_{2}\color{black}))\\
\\
\text{ReLu}(x) = \max(x, 0)
\tanh{x} = \end{align*}
\end{document}